Lubrication of diesel pistons is needed for a variety of reasons: to provide cooling, to control friction and wear, and to remove particulate material such as wear debris. During this process, thin films of lubricant are exposed to high thermal loads which can lead to deposit formation on piston lands and in ring grooves. Excessive piston deposits are undesirable because they tend to interfere with proper ring operation.
In developing lubricating oils that can withstand those high thermal loads, it is important to know exactly what those thermal loads are (i.e., the "steady-state temperature"). By "steady-state temperature," we mean the equilibrium temperature configuration of an engine system, such as in a non-cyclic ASTM standardized lubricant qualifying run. One way of measuring those loads is by measuring the piston hardness and relating hardness to temperature by a suitable calibration curve. (See "Microstructure of Carbonaceous Diesel Engine Piston Deposits," ASLE Preprint No. 87-AM-5D-3, which is incorporated herein by reference for all purposes.)
Although once quite popular, the use of Aluminum-Copper alloys in piston manufacture appears to be waning in favor of Aluminum-Silicon alloys. By "Aluminum-Copper alloy," we mean a mix of materials, predominantly Aluminum, and containing Copper as the principal alloying element. By "Aluminum-Silicon alloy," we mean a mix of materials, predominantly Aluminum, and containing Silicon as the principal alloying element. The Aluminum-Silicon alloys, while exhibiting many superior performance characteristics to their predecessors, do not yield much temperature information by hardness analysis.